In X-ray generating tube for medical applications, a stream of electrons emitted from a cathode and accelerated to high energy in an evacuated envelope strikes an anode target to release electromagnetic energy in the form of X-rays. The tube envelope comprises a window transparent to X-rays so that radiation passes through the window to a patient undergoing examination or treatment.
In many applications, it is desirable to narrowly focus a stream of electrons onto a small area of the anode target, known as the "focal spot". Only a small fraction of the electron energy is converted to X-rays, while most of the electron energy is converted directly to heat energy. Some electrons have enough kinetic energy to leave the anode target and fly off in random directions. These electrons, still subject to the high voltage field, tend to be reabsorbed back into the anode target or any other surface which intercepts their course. These electrons are known as "secondary or stray" electrons as opposed to the electrons in the primary stream from the cathode.
Secondary electrons cause not only undesirable heating of the tube envelope near the focal spot area, but create a so called, "off focal radiation". Off focal radiation produced by secondary electrons creates a background radiation pattern which damages the quality of an X-ray image by increasing the size of the focal spot of the X-ray tube and leading to the geometric blurring.
One approach to control the size of the focal spot of the electron stream on the anode target has been to mount a cathode filament to a focusing support member. In a conventional tube design a cathode usually comprises one or few electrically energized filaments. The filament is mounted into a cup-shaped electrode surrounding the filament on its side opposed to the one facing the anode target. In order to control the size of the focal spot being formed by an electron beam on the anode target, electrical bias voltage has been applied between the cup-shaped electrode and the filament. The cup electrode has operated at cathode potential and was biased negatively when electron beam cut-off has been required.
The improvement of this design and attempt to minimized the size of the focal spot for obtaining a high resolution images resulted in a system which had a second electrode interposed between the cup electrode and anode target. The second electrode was connected to a variable, independent from the cathode, voltage supply. Though these systems provided some improvement in adjusting the focal spot, the use of the independent source for electrode bias voltage brought about undesirable and unpredictable changes in the focusing regulations connected with power line fluctuations and other transients during X-ray exposures.
One attempt to cure this problem by designing the bias voltage circuit for the focusing electrode between the cathode and the ground so that any variations in the cathode voltage would vary the bias voltage in the same proportion did not give the significant improvement in minimizing the focal spot size, while made the tube susceptible to failure due to a high voltage transience.
Different approach to address these problems has been made by Furbee, et al. in the U.S. Pat. No. 5,007,074 entitled, "X-RAY TUBE ANODE FOCUSING BY LOW VOLTAGE BIAS". According to this invention, the cathode cup is battery biased at a low voltage in order to reduce the dispersion of the electron beam which causes the wings on the anode target focal spot. A small, self-contained battery is introduced in the tube between the X-ray tube envelope and the housing, and is used as a bias voltage source. Although satisfactory in certain respects, such a system suffers from disadvantages. The battery will deplete over time and will change its output value over a temperature range conventional to the tube housing. This change will be inversely proportional to the desired optimum value, while the battery bias will remain unchangeable in spite of the changes in the power of the tube.